A rotation mechanism with limited stroke usually operates in conjunction with a limit-angle protection device to prevent a driven apparatus from suffering damage which might otherwise happen if the angle of rotation of the mechanism exceeds the limit. Although a conventional mechanical stopper puts a restraint on the angle of rotation of the mechanism, it is incapable of protective power interruption, thereby causing damage to the mechanism or overloading the device under an overly large driving force.
A conventional limit switch applies to the installation of a rotation mechanism. Once it approaches an angular limit, the rotation mechanism will trigger a lever of the limit switch in breaking the circuit of the rotation mechanism such that the rotation mechanism will stop operating. In general, the modes in which the limit switch operates come in various forms, including a mechanical mode, an electromagnetic code, and an optical mode. Regardless of which mode the limit switch is operating, the limit switch must operate in conjunction with the rotation mechanism, or, to be specific, must come into contact with the rotation mechanism in order to trigger the lever of the limit switch as soon as the rotation mechanism attains a rotational limit-angle.
US20040017187 discloses a sensor of rotational or linear displacement. The sensor comprises magnetic blocks and a magnetic field detection circuit, wherein the angular or linear displacement is determined in accordance with a sinusoidal wave generated from two linear Hall elements (LHE), and the sinusoidal wave features a phase difference of 90 degrees. However, US20040017187 also discloses that the magnetic blocks take on a special shape in order for the linear Hall elements to produce a magnetic field which manifests a position-dependent linear relationship. But the special shape of the magnetic blocks has the following disadvantages: since the journey of the displacement of the magnetic blocks correlates with the size of the magnetic blocks, given a constant installation space, the larger the magnetic blocks, the less the remaining displacement; to achieve a relatively large range of motion, the required volume of the device must increase; what is more, US20040017187 discloses just a single limit-angle surveillance mechanism and thus still has room for improvement in security.